Differential Barothermal Analysis Research Articles

Phase Transformations of 1.4 at % Cu–Al Binary Alloy at High Pressures and Temperatures

Phase transformations of 1.4 at % Cu + 98.6 at % Al alloy have been studied at atmospheric pressure by differential scanning calorimetry and at a moderately high hydrostatic pressure (~100 MPa) by differential barothermal analysis. High pressure has been shown to raise the solvus (θ-Al2Cu + α-Al → α-(Al) solid-state transformation) temperature of the alloy relative to the equilibrium value obtained at atmospheric pressure. According to our estimate, the heat of solid-state θ-phase dissolution in the α-matrix at 100 MPa is four times that at atmospheric pressure. High-pressure crystallization has been shown to have a significant effect on the microstructure of the alloy, with a manyfold increase in the particle size of the θ-Al2Cu intermetallic phase.

Effect of Hot Isostatic Pressing on the Structure and the Mechanical Properties of an Al–7Si–7Cu Composite Alloy

Differential scanning calorimetry and differential barothermal analysis demonstrate that a hydrostatic pressure of 100 MPa leads to a shift in the characteristic temperatures of an aluminum-matrix Al–7 wt % Si–7 wt % Cu composite alloy to higher temperatures at pressure coefficients of 0.04–0.12 K/MPa. An increase in the solidus temperature by 6°C, which is most significant from a practical point of view, is shown to enable one to increase the hot isostatic pressing (HIP) temperature as compared to the homogenizing annealing temperature or the heating temperature for quenching. It is also shown that HIP treatment (the temperature of isothermal holding is 515 ± 5°C, the hydrostatic pressure is 100 MPa, the holding time is 3 h) leads to a more compact morphology of Al2Cu and Si eutectic particles in the alloy structure. The results of studying the mechanical properties after HIP treatment and hardening heat treatment according to regime T6 (quenching from 505°C for 1 h + aging at 175°C for 6 h) show that the strength of the alloy increases from 335 to 360 MPa, the relative elongation increases by ~35% (from 2.0 to 2.8%) as compared to a usual specimen, and the reproducibility of the mechanical properties is noticeably improved.

Analysis of the Effect of Hydrostatic Pressure on the Nonvariant Eutectic Transformation in Al–Si, Al–Cu, and Al–Cu–Si Systems

Differential barothermal analysis (DBA) is used to analyze the effect of hydrostatic pressure of 100 MPa on characteristic temperatures of a number of eutectic alloys (wt %), such as Al–10Si, Al–12Si, Al‒22Cu, Al–33Cu, and Al–7Cu–7Si. According to DBA data, the increase in pressure resulted in an increase in the temperatures of phase transformation; in practice, the most important of them is the nonvariant eutectic transformation temperature that determines the solidus of alloys. It was found that, for the binary systems, the temperatures of the nonvariant eutectic transformation L → (Al) + Si and L → (Al) + Al2Cu increase by 6 and 11°С (from 577 to 583°С and from 548 to 559°С, respectively); for the ternary system, the temperature of transformation L → (Al) +Al2Cu + Si increases by 6°С (from 520 to 526°С). Theoretical analysis, performed using thermodynamic models and Thermo-Calc software, shows that the increase in the eutectic transformation temperature with increasing pressure is directly dependent on the relative decrease in the molar volume of system upon associated eutectic transformation. In this case, the excess dissolution of silicon in (Al) under high pressure can lead to an additional decrease in the molar volume of the system, whereas the increase in the copper solubility is thermodynamically unfavorable.

Effect of Barothermal Processing on the Solid-State Formation of the Structure and Properties of 16 at % Si–Al Hypereutectic Alloy

We describe barothermal processing (hot isostatic pressing) of a 16 at % Si–Al binary alloy for 3 h at a temperature of 560°C and pressure of 100 MPa for 3 h, in combination with measurements of heat effects during cooling. The results demonstrate that this processing leads to the fragmentation of the silicon structural constituent and ensures a high degree of homogenization of the as-prepared alloy. Heat treatment of the 16 at % Si–Al alloy at 560°C and a pressure of 100 MPa leads to a thermodynamically driven enhanced silicon dissolution, up to ~10 at %, in the aluminum matrix, resulting in the formation of a supersaturated solid solution, which subsequently decomposes during cooling. We analyze the complete porosity elimination process, which makes it possible to obtain a material with 100% relative density. According to differential barothermal analysis, microstructural analysis, and scanning and transmission electron microscopy data, barothermal processing of the 16 at % Si–Al alloy produces a bimodal size distribution of the silicon phase constituent: microparticles 3.6 μm in average size and nanoparticles down to ~1 nm in diameter. The Al matrix has been shown to contain a high density of edge dislocations. Barothermal processing reduces the thermal expansion coefficient and microhardness of the hypereutectic alloy. We conclude that solid-state barothermal processing is an effective tool for completely eliminating microporosity from the 16 at % Si–Al alloy, reaching a high degree of homogenization, and controlling the microstructure of the alloy, in particular by producing high dislocation density in the aluminum matrix.

High-pressure phase transitions and structure of Al–20 at % Si hypereutectic alloy

Phase transformations of an Al–20 at % Si high-silicon hypereutectic alloy have been studied by differential barothermal analysis at temperatures of up to 800°C in argon compressed to 100 MPa. High pressure has been shown to raise the melting point of the alloy by 5°C during heating and to lower the eutectic solidification temperature by 5°C during cooling in comparison with the canonical phase diagram of the Al–Si system. At a temperature of 553°C, heating and cooling lead to silicon dissolution and decomposition of the aluminum-based solid solution, respectively. After high-pressure solidification, the silicon particles in the alloy have a bimodal size distribution. Quantitative porosity characteristics in the alloy after a barothermal scanning cycle are similar to those in the as-prepared alloy. The lattice parameters of the silicon and aluminum remain unchanged. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.

Barothermography and microstructure of the hypoeutectic and eutectic alloys in Al–Si system

Differential barothermal analysis (DBA or HP-DTA) of the phase transformations in hypoeutectic Al-10 at.% Si and eutectic Al-12 at.% Si alloys was carried out at 100 MPa of argon pressure and at temperatures up to 700 °C. Liquidus temperature of Al-10 at.% Si is reduced during melting and crystallization by 3–4 °C as compared with the atmospheric pressure data. Increase in the eutectic transformation L → (Al) + Si temperature up to 580–581 °C during heating was found for the Al-10 at.% Si and Al-12 at.% Si alloys. According to the barothermography data, a slight thermal effect at 553 °C is detected for both alloys, which was identified as the decomposition of aluminum-based solid solution and precipitation of silicon particles. The improvement of silicon phase morphology for Al-10 at.% Si and Al-12 at.% Si alloys after hot isostatic pressing (HIP) at the pressure of 100 MPa and temperature of 550 °C was found. No porosity in the alloy has been established after HIP treatment. After HIP thermal expansion, coefficients of the alloy are noticeably decreased by 25 (Al-10 at.% Si) and 50 % (Al-12 at.% Si).

Pressure effect on the phase transformations and structure of an Al-12 at % Si alloy

Phase transformations of an Al-12 at % Si alloy have been studied by differential barothermal analysis at temperatures of up to 700°C in argon compressed to 100 MPa. Heating at a rate of 8°C/min under a pressure of 100 MPa has been shown to increase the temperature of the L → (Al) + Si eutectic transformation in the alloy by 4°C. After crystallization of the binary eutectic, the differential thermal analysis cooling curve showed an additional exothermic peak, corresponding to the decomposition of the aluminum-based solid solution (Al) at 547°C and precipitation of silicon particles in the nanometer range. A barothermal scanning cycle reduced numerical porosity characteristics of the alloy, except for the pore number density. The lattice parameter of silicon microparticles in the alloy approaches values in the literature, whereas that of nanoparticles is slightly greater. The lattice parameter of the aluminum remains unchanged during crystallization and cooling in compressed argon. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.

Phase transformations in a binary 10 at % Si-90 at % Al alloy at high pressures and temperatures

Differential barothermal analysis of the phase transformations in a 10 at % Si-90 at % Al alloy has been performed at temperatures up to 700°C in an argon atmosphere compressed to 100 MPa. A slight change of the eutectic melting/solidification temperature is found. The liquidus temperature of the alloy, which is determined upon melting and solidification, coincides with that determined at atmospheric pressure. At 551–554°C, an aluminum-based solid solution decomposes with the formation of silicon nanoprecipitates. The porosity of the alloy after barothermal analysis is almost unchanged. The lattice parameters of micron-sized silicon particles decrease, whereas those of nanoparticles increase relative to the tabulated parameters. The lattice parameters of aluminum subjected to solidification and cooling in a compressed argon medium decreases. The micorhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.

High-pressure phase transformations, microstructure, and magnetic properties of the hypereutectic alloy 10Ni-90Al

Phase transformations of the hypereutectic alloy 10Ni-90Al have been studied by differential barothermal analysis at temperatures of up to 900°C and pressures of up to ≅100 MPa. We have determined the pressure coefficients of the liquidus and solidus temperatures and the temperatures of solid-state dissolution and precipitation of the intermetallic phase Al3Ni. At high pressures, the solid-state decomposition of the supersaturated solid solution of nickel in aluminum occurs at 622°C and yields an (Al) + Al3Ni mixture. We assume that, during cooling of the alloy at high pressure, intermetallic particles form in three steps. Barothermal analysis data have been compared to the canonical t-x phase diagram of the Al-Ni system. As a result of melting and crystallization at high pressure, the microcrystalline structure of the as-prepared alloy transforms to a macrocrystalline, dendritic structure and the micropore concentration increases. X-ray diffraction data indicate an increase in the unit-cell volumes of the Al and Al3Ni in the alloy crystallized at 100MPa. We have studied the magnetic properties of the alloy after barothermal analysis.

Barothermal analysis of phase transformations of an Al-15 at % Ni alloy and its structure and magnetic properties

Phase transformations of the 15Ni-85Al alloy have been studied by differential barothermal analysis at temperatures of up to 900°C and pressures of up to ≅ 100 MPa. We have determined the pressure coefficients of the liquidus and solidus temperatures and the temperatures of solid-state dissolution and precipitation of the intermetallic phase Al3Ni in the Al matrix. Melting and crystallization in compressed argon have been shown to increase the micropore concentration in the material by about one order of magnitude relative to the as-prepared alloy. Crystallization of the alloy at 100 MPa has a significant effect on its microstructure and the particle size and morphology of the intermetallic phase in the aluminum matrix and increases the unit-cell volumes of the Al and Al3Ni. We have compared the magnetic properties of the alloy before and after barothermal analysis.

Transformation in metal materials during hot isostatic pressing

The paper presents a review of transformations in a number of metal materials during hot isostatic pressing which result in changes in the shape, density, morphology, and composition of the structural components of some metals, including cast, powder, and composite titanium alloys, heat-resistant nickel alloys, hard alloys of the WC-Co system, and also some aluminum and aluminide materials. Methods for studying the compaction processes using an eddy current dilatometric cell and differential barothermal analysis to determine the baric shift of the characteristic temperatures for some heat-resistant nickel alloys are described. The data on the practical application of hot isostatic pressing are presented, which makes it possible to fabricate the most important parts of gas turbines. It is shown that hot isostatic pressing causing transformations of different physicochemical origin should be considered as a necessary step of the production of metals to ensure their limiting strength characteristics.

Phase transformations at high pressures and temperatures and the structure of a hypoeutectic 1Ni-99Al alloy

The phase transformations in a hypoeutectic 1Ni-99Al alloy are studied by differential barothermal analysis in the temperature range up to 750°C at a compressed argon pressure up to ∼100 MPa. The Al matrix of the initial alloy is found to be saturated by micropores at a concentration of 3.7 × 1010 cm−3. After melting and solidification in a compressed argon atmosphere, the micropore concentration increases to 3.2 × 1011 cm−3. As a result of melting and solidification at a high pressure, the initial fine-grained structure of the alloy with an average grain size of 16 μm transforms into a coarse-grained structure during dendritic solidification. The processing of electron-microscopic images is used to determine the volume content of intermetallic compound Al3Ni in the Al matrix. The liquidus temperature of the alloy at 100 MPa increases by 10°C, and the solidus temperature is 5°C higher than the eutectic transformation temperature in aluminum-rich Al-Ni alloys. The solid-phase decomposition of the supersaturated solid solution of nickel in aluminum occurs at 630°C. At 100 MPa, the field of solid solutions of nickel in aluminum extends to 1.2 at % Ni as compared to the Al-Ni system at atmospheric pressure. The lattice parameters of Al and Al3Ni are found to increase in the alloy solidified at 100 MPa. The microhardness of the Al matrix in the alloy is measured after a barothermography cycle. A portion of the Al-Ni phase diagram is proposed for a pressure of 100MPa in the nickel content range 0–4.3 at %.

Barothermal analysis and structure of the eutectic Al-Ni (2.7 at % Ni) alloy

Phase transformations of the 2.7Ni + 97.3Al eutectic alloy have been characterized by differential barothermal analysis (DBA) at temperatures of up to 750°C and pressures of up to ∼100 MPa. The results demonstrate that the Al matrix of the as-prepared alloy was saturated with micropores. After melting and crystallization in compressed argon, the micropore density increased. As a result of melting and solidification, the fine-grained structure of the as-prepared alloy transformed to a macrocrystalline, dendritic structure. Electron microscopy was used to determine the volume fraction of the intermetallic phase NiAl3 in the Al matrix. Supersaturated solid solutions of nickel in aluminum decompose at 626°C to give a mixture of Al〈Ni〉 and NiAl3. At 100 MPa, the nickel-aluminum solid-solution range extends to 2.7–2.8 at % Ni, and the nickel content at the eutectic point may reach 3.1–3.3 at %. The Al and NiAl3 in the alloy solidified at 100 MPa had reduced lattice parameters. We determined the microhardness of the Al matrix after DBA.

Thermography of the phase transformations in nickel-based eutectic alloys at high pressures and temperatures

The phase transformations in STEMET 1301A and STEMET 1311 nickel solder alloys are studied by differential barothermal analysis and metallography at temperatures up to 1100°C and pressures up to 120MPa. When the pressure increases, the temperature of the onset of crystallization of the initial amorphous STEMET 1301A alloy decreases and that of the STEMET 1311 alloy increases. At high pressures, the solidus temperatures of the alloys changes insignificantly and the liquidus temperature of the STEMET 1301A alloy increases by 30°C. The solidification ranges of both alloys at 120 MPa are found to broaden substantially.

Thermography of the phase transformations in a hypoeutectic Al-7% Si-0.5% Mg silumin at high pressures and temperatures

The phase equilibria in a hypoeutectic AK7 aluminum alloy (Al-7% Si-0.5% Mg silumin) are studied by differential barothermal analysis in the pressure range 5–131 MPa at temperatures up to 960 K. The baric dependences of the liquidus temperature during heating and the solidus and liquidus temperatures during cooling are found to have a minimum at 5 MPa. The solidus temperature during heating increases sharply as the pressure increases from 5 to 53 MPa and remains almost unchanged at a pressure p ≥ 53 MPa. The baric coefficients of the solidus and liquidus temperatures in the pressure range 5 ≤ p ≤ 53 MPa achieve ~0.6 K/MPa, which is much higher than the baric coefficient of the melting temperature of pure aluminum. A pT phase diagram is proposed for the AK7 alloy.

Thermographic investigation of the phase equilibria in an aluminum-based alloy at high pressures and temperatures

The phase equilibria in a commercial aluminum alloy are studied by thermography, particularly, by differential barothermal analysis, in the pressure range from 10 to 150 MPa at temperatures below 960 K. The liquidus temperature is found to increase insignificantly with the pressure, and the solidus temperature increases jumpwise at a pressure of 60 MPa. The phase pT diagram of the aluminum alloy and a eutectic model of the baric dependences of the solidus and liquidus temperatures are proposed.

Differential barothermal analysis in the course of reactive powder barothermal processing of RuAl alloys

Thermostable high-temperature structural alloys based on the refractory (melting temperature Tm=2060°C) RuAl intermetallic (IM) with an ordered B2 crystal structure (CsCl type) are developed. This IM surpass other aluminides (NiAl, TiAl and Ni3Al) used as the base for the development of high-temperature alloys and matrices of high-temperature composites (CM) intended for hot parts of supersonic engines, which serve at the temperatures exceeding operation temperatures of modern nickel-base superalloys. The differential barothermal analysis was used to develop the basic technological process of barothermal reaction sintering to produce near-net shape billets from RuAl-based structural materials.

Pressure effect on the phase transitions in Ni-base multicomponent systems

The phase transformations in the three Ni-base alloys at the temperature range up to 1500°C and at the pressure of 170 MPa were studied. Solidus, γ’-phase dissolution/precipitation, carbides dissolution/precipitation and liquidus temperatures are established. Multiple exceeding of the Ni-base alloys characteristic temperatures pressure coefficients over metal components ones (up to 10 times) is observed both at the heating and cooling scans during the experiments on the differential barothermal analysis. On the DBA curves had been obtained at 170 MPa, there are from two to three peaks, which connected with the carbides dissolution/precipitation, while on the each conventional DTA curve there was merely one ‘carbide’ thermal effect. The liquidus temperature pressure coefficients correlate with the Ni melting point pressure coefficient.

Pressure Effect on the Phase Transformations of a Nickel–Aluminum Alloy

Differential barothermal analysis is used to study the pressure effect (≤180 MPa) on the temperatures of phase transformations in a model nickel–aluminum alloy. The pressure coefficients for these transformations are larger than that for the melting point of pure nickel by a factor of 3–5.

Differential barothermal analysis (DBA) of Ni-base alloys

The technique of differential barothermal analysis (DBA) combines the hot isostatic pressing technique and classical differential thermal analysis has been elaborated. This technique allows the study of the phase equilibria in the most of inorganic systems under pressure up to 200 MPa and temperatures up to 2000°C. The Ni-base alloy with the solidus, liquidus, g-phase and carbides dissolution temperatures of 1328, 1390, 1272 and 1255°C, accordingly, was chosen as a model material. By DBA technique the pressure coefficients of the above mentioned temperatures were established. The values of the obtained coefficients are 3-5 times larger than the melting point pressure coefficient for the pure Ni (the alloy base metal). The discussion of the data obtained is carried out.